Some X58 mobos have 4 or more x16 slots. Until recently, they've only been for video cards. Now there is an x16 drive controller, too. More of such things to come? http://www.computerworld.com/s/article/9188758/OCZ_invents_drive_interface_3X_faster_than_SAS FWIW...
I've always liked OCZ. They're never cheap on their newest technology but they're good at what they do. I forget how many thousands of dollars I paid for my first four chips of DDR3 but it was more than half the cost of the whole system in a Quad Core Extreme cpu system.
History shows that a lot of devices first appear as an add-on because the main board providers did not think of it... and then later on got absorbed into the mother board. e.g. modem (if you still use those devices these days) NIC card sound card (now MoBo even has 5.1) video (now MoBo offers 1 built-in) disk drive controller (IDE, SCSI, SATA) USB connection (now MoBo offers 6) etc. etc.
The news isn't, "here's an add-on device for the mobo"... it's that some device besides a video card needs an x16 slot to take advantage of the high bandwidth.